Selected Quarterly Financial Data (Schedule of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Data [Abstract]
Revenues	$ 178,785	$ 141,047	$ 166,186	$ 124,499	$ 146,769	$ 128,876	$ 158,805	$ 131,945	$ 135,462	$ 109,318	$ 155,477	$ 115,618	$ 610,517	$ 566,395	$ 515,875
Operating income	40,038	16,931	26,888	23,597	40,628	2,285	53,635	39,174	50,424	28,602	62,795	33,019	107,454	135,722	174,840
Net income	36,837	10,323	17,713	10,433	27,927	(5,000)	40,591	29,994	47,372	21,247	45,846	24,110	75,306	93,512	138,575
Net income (loss)	$ 36,754	$ 10,261	$ 17,635	$ 10,377	$ 27,853	$ (5,137)	$ 39,710	$ 29,406	$ 46,529	$ 20,825	$ 45,083	$ 23,777	$ 75,027	$ 91,832	$ 136,214
Basic, Per share amount	$ 0.34	$ 0.10	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.28	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.70	$ 0.86	$ 1.27
Diluted, Per share amount	$ 0.34	$ 0.09	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.27	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.69	$ 0.85	$ 1.27